The silverleaf whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae), is an important insect pest of melon and other cucurbit species. Infestation can cause severe yield loss, both directly through feeding damage cause by the whiteflies and larvae, as well as indirectly through sugary excretions serving as substrate for sooty mold and/or transmission of pathogenic viruses, such as CYSDV (Cucurbit Yellow Stunting Disorder), CVYV (Cucurbit Vein Yellowing Virus) or MYaV (Melon Yellowing Associated Virus), and bacteria.
To control B. tabaci insecticides, such as imadacloprid, are frequently applied and breeding for resistance is required to reduce insecticide use.
Sauvion et al. (2005, J Econ Entomology 98:557-567) identified three accessions with potential antibiosis resistance to B. tabaci biotype B, P1161375, P1414723 and P1532841.
Boissot et al. (2003, HortScience 38(1): 77-80) identified low levels of field resistance in three Indian Accessions (P1414723, PI1164723 and 90625) and one Korean accession (PI1161375).
Boissot et al. (2010; Theor Appl Genet 121: 9-20) mapped QTLs (Quantitative Trait Loci) for B. tabaci biotype B resistance, affecting the number of progenies produced by one female of biotype B (antibiosis resistance), to linkage group VII, for one whitefly clonal population, and to linkage group IX, for a different whitefly clonal population. The QTLs were only minor (LOD value 3.6 and 4.0) and the resistance allele for both of these minor QTLs originated from the Korean accession P1161375.
The genetics of whitefly resistance are still largely unknown, most likely to the difficulties to manage whiteflies in the laboratory and difficulties in establishing reliable assays for whitefly resistance.
Tests that are used to identify resistance are either free-choice tests or non-choice tests or field tests.
In free choice assays the insects can choose among different plant genotypes for feeding and reproduction. Such free choice-tests are used to identify antixenosis (non-preference) resistance, i.e. resistance caused by factors that make a plant genotype less attractive. See e.g. Palomares-Rius et al. (2007, Cucurbit Genetics Cooperative Report 30:23-25) who used a free-choice leaf disc assay to show that PI414723 is rejected quickly by whiteflies and that therefore a antixenosis mechanism may exist in this accession in addition to the antibiosis mechanism detected by Sauvion et al. (2005, supra).
In non-choice tests for resistance the insects cannot choose among different plant genotypes for feeding and reproduction, but are only allowed to feed and reproduce on one genotype. This type of resistance affects the insects biotic potential, e.g. they die, produce fewer offspring or grow more slowly (antibiosis or antibiotic resistance).
It is an object of the invention to provide whitefly resistance sources and a genetic region comprising the resistance locus, or a part thereof, which confer resistance, especially antixenosis (non-preference) resistance and field resistance, against whiteflies, B. tabaci biotype B. It is a further object of the invention to provide cultivated melon plants (Cucumis melo L.) and cells, tissues, fruits and other parts of such plants comprising in their genome a whitefly resistance-conferring locus (or a resistance-conferring part thereof), either in homozygous or heterozygous form, whereby the melon plants are resistant against whitefly. Also seeds from which whitefly resistant melon plants can be grown are an embodiment of the invention.
In a further aspect molecular markers are provided, which can be used to detect the presence of and/or to transfer the B. tabaci resistance-conferring locus, or a resistance-conferring part thereof, in/into plants or plant cells of Cucumis melo L using e.g. traditional breeding techniques (such as marker assisted selection). One or more of the markers can, thus, for example be used to transfer the resistance locus, or a resistance-conferring part thereof, into cultivated melon plants which are susceptible to B. tabaci. In one embodiment the resistance locus, or resistance-conferring part thereof, is the locus on chromosome 11 as found in seeds deposited under accession number NCIMB 41965 or NCIMB 41966. In a different embodiment the resistance locus, or resistance conferring part thereof, is the locus on chromosome 11 as found in seeds deposited under accession number NCIMB 42221 or NCIMB 42220. In a further embodiment the resistance locus or resistance-conferring part thereof is the locus on chromosome 11, or a resistance-conferring part thereof, as found in other wild melon plants or wild relatives of melon.
One or more of the markers linked to, or genetically and physically associated with, the B. tabaci resistance locus, or resistance conferring part thereof, can also be used to identify new B. tabaci-resistance sources on chromosome 11, such as other wild accessions of Cucumis melo or wild relatives of melon comprising a B. tabaci-resistance locus on chromosome 11 and for transferring (introgressing) the resistance locus, or a B. tabaci-resistance conferring part thereof, from such accessions into cultivated melon plants. The B. tabaci resistance conferring quantitative trait locus (QTL) on chromosome 11 (equivalent to ICuGI Linkage Group XI, or LG XI) was named Wf_11.1 (for whitefly chromosome 11, QTL 1).
WO2014/031770 describes a QTL for ZYMV/WMV resistance introgressed onto chromosome 11, but this QTL maps to a different region of chromosome 11, especially to the other half of chromosome 11 than the QTL of the instant invention.